I see that the Sony support page is virtually useless for what you are looking for.
I recommend that you take the Sony to a local PC service shop that you trust.
Have them open up the case and help determine what your hard drive options are.
It is easy to tell from the connectors what type of drive you have, and then determine the physical options of the drive bay.
The Intel chip-set and the BIOS "should" support drives up to 120GB with no special requirements.
For larger drives, you may need to use the drive manufactures setup utility.
Your next issue is going to be installing Windows on the new drive and getting your files moved.
If you have the actual recovery CDs you are all set. Otherwise, you may want to have the service shop transfer your existing installation to the new drive and install the drive in your laptop.
If you're asking for technical help, please be sure to include all your system info, including operating system, model number, and any other specifics related to the problem. Also please exercise your best judgment when posting in the forums--revealing personal information such as your e-mail address, telephone number, and address is not recommended.
upgrade hard drive for sony vaio PCG TR3A